1. Field of the Invention
The present invention relates to a device to press a frame for a apertured grill which constitutes a color selector of a color cathode ray tube (abbreviated as CRT hereinafter).
2. Description of Related Art
An apertured grill is provided as a color selector in a trinitron type CRT which is used for television receivers and television monitors of computers.
As shown in FIG. 7, an apertured grill 1 is provided with a frame 101 formed in a shape nearly rectangular and a grill 107 spread tightly inside the frame 101.
The frame 101 comprises top and bottom horizontal members 103 and right and left vertical members 105 connecting to right and left ends of the horizontal members, and the horizontal members 103 are curved so that the distance between the top and bottom horizontal members is wider at the middle of the horizontal members between right and left ends thereof than that at the right and left ends of the horizontal members, and curved so that the right and left ends of the horizontal members are in the front and the middle between right and left ends in the rear.
On the grill 107 many vertically long slits 109 are provided side-by-side in the horizontal direction with a certain interval, and the grill 107 is tensioned tightly inside the frame 101 by welding a sheet flat apertured grill 3, on which the slits 109 are formed, on the front sides of the vertical members 105 as shown in FIG. 8 and then by removing the outside portion of the flat apertured grill 3 wherein the outside portion means the portion extending outside beyond the frame 101.
The flat apertured grill 3 is welded on the horizontal members 103 as described herein under. The top and bottom horizontal members 103 are forced to deform so as to get near each other, and a flat apertured grill 3 which is being extended vertically is placed on the front side of the frame 101, and the apertured grill 3 is welded so that the grill 107 does not slack between the slits 109 after completion of the apertured grill 1.
FIG. 9 shows schematic structure of a conventional pressure device for pressing and deforming the top and bottom horizontal members 103.
The pressure device 41 is provided with upper and lower pressure mechanisms 43 and 45 for pressing the top and bottom horizontal members 103 from the outside of the frame 101 respectively.
The upper pressure mechanism 43 has a fixed pressure lever 4301 provided above the top horizontal member 103, and the lower pressure mechanism has a slide shaft 4501 movable in the direction to get near to and get far from the bottom horizontal member 103 and a movable pressure lever 4503 supported rotatably at the top end of the slide shaft 4501 and provided under the bottom horizontal member 103.
At both the right and left ends of the pressure levers 4301 and 4503 of the upper and lower pressure mechanisms 43 and 45, pressure blocks 4303 and 4505 are respectively supported rotatably, and on both the right and left ends of the pressure blocks 4303 and 4505, pressure tips 4305 and 4507 which are engageable with the top and bottom horizontal members 103 are provided upward.
When the top and bottom horizontal members 103 are pressed using the pressure device 41, the pressure lever 4503 of the lower pressure mechanism 45 is moved toward the upper pressure mechanism 43 through the movement of the slide shaft 4501, and during the movement, the pressure tips 4507 of the pressure lever 4503 are brought into contact with the bottom horizontal member at four positions of two places on the right and two places on the left with a certain distance between the twos, and the pressure tips 4305 of the upper pressure mechanism 43 are brought into contact with the top horizontal member 103 at four positions of two places on the right and two places on the left with a certain distance between the twos, and the pressure lever 4503 of the lower pressure mechanism is moved further toward the upper pressure mechanism 43, thus the further movement causes the pressure to press the horizontal members 103.
Then, the rigidity of the bottom horizontal member 103 is higher at the right and left end portions of the bottom horizontal member 103 than at the middle portion between right and left ends of the bottom horizontal member 103 because at the right and left ends of the bottom horizontal member 103, the bottom horizontal member 103 is connected with the vertical members 105, consequently after the contact of the pressure tips 4305 and 4507 with the bottom horizontal member 103, the pressure tips slide from the middle portion between right and left ends, where the rigidity is low, to both the end directions of the horizontal members 103.
However, because the conventional pressure device described herein above is so constituted that only the pressure lever 4503 of the lower pressure mechanism 45 is supported rotatably out of upper and lower pressure levers 4301 and 4503 and only the pressure lever 4503 is made to move toward the upper pressure mechanism 43, when the top and bottom horizontal members 103 are pressed, the mechanism of the lower pressure mechanism 45 in which the pressure tips 4507 contact with the bottom horizontal member 103 while the pressure lever 4503 is rotating and moving is different from the mechanism of the upper pressure mechanism 43 in which the pressure tips contact with the top horizontal member 103 while the pressure lever 4301 is fixed, that is, the position of contact point between the pressure tips 4305 of the upper pressure mechanism 43 and the upper horizontal member 103 is different from the position of contact point between the pressure tips 4507 of the lower pressure mechanism 45 and the lower horizontal member 103.
Since the pressure tips 4305 and 4507 are, conventionally, so structured as to slide on the horizontal members 103 when pressuring the horizontal members 103, the difference in rigidity depending on the position of contact point between the pressure tips 4305 and 4507 and the horizontal members 103 causes the difference in frictional force between the friction of the top horizontal member 103 with the pressure tips 4305 and the friction of the bottom horizontal member 103 with the pressure tips 4507, and the difference in frictional force makes a difference in sliding of the pressure tips 4305 and 4507 on the horizontal members 103, and thus the position of contact point of the pressure tips 4305 with the upper horizontal member 103 is different from the position of contact point of the pressure tips 4507 with the bottom horizontal member 103.
The difference in position causes the difference in pressing condition between the top horizontal member 103 and the bottom horizontal member 103, and the difference in pressing condition causes the twisting of the frame 101 and the grill 107 welded on the top and bottom horizontal members 103, and the twisting causes the slacking of the grill 107 between the slits 109, the slacking is significantly disadvantageous.
In addition, since the conventional mechanism is so structured that the pressure tips 4305 and 4507 contact with the horizontal members 103 at positions separated toward both ends of the horizontal members with a certain wide space, the positioning of contact points near the ends causes the stress concentration at the contact points of the horizontal members 103 with the pressure tips 4305 and 4507, which causes the unbalanced stress on the horizontal members between contacting portion with the pressure tips 4305 and 4507 and non-contacting portion with the pressure tips 4305 and 4507. Thus, the horizontal members 103 are pressed unbalancedly, and the unbalanced stress causes the slacking of the welded grill 107 between the slits 109. Then, the slacking is significantly disadvantageous.